The present invention relates to a method which allows for the selective plating of a metal substrate, such as an electrical contact. The selective plating thereof, such as by plating with a precious metal, is achieved herein by first applying to all surfaces of such contact a thin coating of a sprayable styrene-acrylic co-polymer resist, drying the resist, preferably followed by the selective removal thereof by a technique called laser ablation, precious metal plating of such selective areas, and removal of the remaining resist from such contact. However, the plating resist of this invention is also applicable to other plating methods such as by the use of a mask or other means for selective plating as known in the art.
A preferred embodiment of this invention is the selective plating of electrical terminals. Typically, such terminals are stamped and formed from metal strip and are attached to a carrier strip which is useful for strip feeding the terminals through successive manufacturing operations. One necessary manufacturing operation involves plating; i.e., electroplating, the electrical contact surfaces of the strip fed terminals with precious metal or semi-precious metal, such as gold or alloys thereof. Such metals are characterized by good electrical conductivity and little or no formation of oxides that reduce said conductivity. Therefore these metals, when applied as plating, will improve conductivity of the terminals. However, the high cost of these metals has necessitated precision deposition onto the contact surfaces of the terminals, and not on surfaces of the terminals on which plating is not necessary. By being able to initially control the selective resist removal procedure, one can ultimately control and limit the application of the precious metals.
Ablation is defined as the process of removal of a part such as by melting or vaporization. The laser is the mechanism by which one may achieve the selective melting or vaporization. By the use of different lasers, particularly ones utilizing a broadly differing wavelength, the general process of laser ablation is affected. For example, by the use of an excimer laser, operating in the U.V. range, coupled with a resist essentially transparent to the wavelength of such laser, and a metal substrate from which the resist is to be removed and which absorbs such wavelength, as taught in co-pending application, Ser. No. 180,417, a different laser assisted material removal process is observed. The latter process, which is more energy efficient, may be termed "patch blowoff." That is, rather than melting or vaporizing the resist, an interesting phenomena occurs at the resist/substrate interface resulting in the overlying resist being blown off, essentially as solid particles. Thus, while laser ablation has been broadly used to define any process where a laser is used to assist in a material removal process, it will be appreciated that the certain parameters applied will render the various approaches quite distinctive.
Returning now to the broad concept, it can be acknowledged that selective removal of a resist may be accomplished by the technique known as laser ablation. Reports have appeared in the literature regarding attempts at laser ablation of polymer coatings on metals, and regarding methods of multi-shot removal of polymer coatings on non-metals. R. Srinivasan et al, in the JAP 59, 3862 (2986) and JVST, B1, 923 (1983) describe, for example, the use of excimer laser wavelengths which are strongly absorbed directly in the polymer itself to achieve removal of polymer by chemical bond-breaking or heating to vaporization, or a combination of both. However, the authors found that polymer ablation occurs when the laser light is absorbed within about the first 0.2 micron or less of the polymer surface. Then only that polymer material within the characteristic absorption depth was removed In order to remove a thicker polymer film, such as is necessary for most electroplating requirements, multiple laser shots would be required. The use of multiple shots is much less desirable than single shot removal One problem associated with the method of Srinivasan et al, wherein the laser light is directly absorbed in the polymer, is that choosing a laser wavelength too strongly absorbed in the polymer necessarily implies a small absorption depth and small thickness removed. On the other hand, choosing a wavelength too weakly absorbed in the polymer precludes depositing sufficient energy per unit volume of polymer to achieve ablation. The compromise value between these extremes dictates that no more than about 0.3 micron per pulse can be removed in the best case. Cole et al, in Mat. Res. Soc. Symp. Proc. 72, 241 (1986), concur with Srinivasan et al in this finding The above process represents the current state of the art on excimer laser ablation of polymers.
In U.S. Pat. No. 4,671,848 to Miller et al, a method for the removal of a dielectric coating from a conductor, by means of a focused, high energy radiation source, is taught. More particularly, in said method a laser source is focused to a point above the coating which results in a plasma ionized region being formed. As a consequence, the coating is removed in a preselected region on the underlying conductor. In other words, the laser ablation depends on absorption of laser light by ionized air or other plasma and transmission to the dielectric. A difficulty of this method is the ability to control and adjust the air breakdown so as to ensure there is no damage to the conductor, i.e. underlying substrate, and to achieve removal of the residual layer. Another difficulty is that only a small area corresponding to the tight focus region can be removed on each shot. Miller et al state that extended areas are to be ablated by multiple shots while moving the workpiece, or the laser focus.
Notwithstanding such prior art teachings, co-pending application, Ser. No. 180,417 discovered, among other features, a more energy efficient laser process alternative to complete ablation of the material being removed. A single excimer laser shot which is not appreciably absorbed by the coating material penetrates through the entire thickness of the coating and causes heating of the metal substrate surface. The heated metal substrate causes vaporization of a thin layer of the coating material next to the metal, thereby destroying the bond of the coating to the metal and providing expanding gasses which cause the blowoff of a patch of coating material covering the total area of the exposed region. The entire thickness of the coating in the patterned exposure area is thereby removed with only a single laser shot with the utilization of only a fraction of the energy required to volatilize the entire volume of the material so removed.
Thus, an important recognition to come from the work of such co-pending application was the criticality in correlating the operating parameters and characteristics of the laser with the properties and characteristics of the plating resist and the properties and characteristics of the underlying metal substrate. It is known from such prior work that the resist must be sufficiently transparent to allow most of the laser radiation to pass through without appreciably decomposing the coating so as to provide heating of the metal surface in the intended ablation patch area.
However, it has now been discovered that additional attributes are necessary. Through continued investigation, the present invention has found that in order for a resist coating to be removed by the above mentioned energy efficient process, it must have additional attributes such that the resist coating will:
a) soften enough when heated by the laser heated metal substrate surface to allow stretch tearing at the patch peripheral boundary between heated and non-heated material to insure patch removal; PA0 b) not soften so much when heated by the laser heated metal substrate surface so that "balloon burst" failure occurs, i.e. rupture tear failure of the center of the ballooning heated patch first, thus relieving blister pressure and allowing the patch edges to fall back into the intended ablation patch area; PA0 c) adhere strongly to the metal substrate so that the resulting resist sidewall bordering the removed areas maintains a sharply defined profile with no undercutting or breaking thereof; PA0 d) provide excellent edge coverage; and PA0 e) possess a high flash point, i.e. outside flammable category. PA0 a) be sufficiently transparent to allow most of the laser radiation to pass through the coating so as to provide heating of the metal surface in the intended ablation patch area; PA0 b) gasify enough when in contact with the laser heating metal substrate surface to provide an expulsion means for the patch being removed; PA0 c) adhere strongly enough to protect the covered surface during plating operations; PA0 d) not deteriorate in the plating bath chemical environment; PA0 e) be strippable after the plating operation is completed; and PA0 f) be able to be applied by spraying or printing. PA0 a) handling characteristics by being classified as non-flammable, PA0 b) edge coverage of object being resist coated, PA0 c) adhesion to the substrate, and PA0 d) maintenance of resist edgewall profile integrity along removed patch boundaries;
as well as preserve the previously established requirements that the resist coating will:
As a result of the improved formulation, the plating resist hereof provides for a superior performance, the manner of which will become apparent from a reading of the specification which follows.